This invention relates to an improved method for the post-exposure of polymeric printing plates. More particularly, this invention relates to post-exposing polymeric relief printing plates, where the improvement comprises postexposing the plate to actinic radiation while the plate is immersed in a dilute aqueous solution of a persulfate salt and a sufficient amount of a water-soluble carboxylic acid to produce a pH of about 1.5 to about 2.5.
It is known in the printing art to provide for the rapid preparation of lightweight polymeric relief printing plates. Such relief printing plates can be prepared by exposing a layer of photosensitive polymeric composition to actinic radiation through a transparency bearing the image which it is desired to reproduce, washing away or otherwise removing the unexposed soluble photosensitive composition leaving an insolubilized relief image. Especially in the case of relief printing plates prepared from liquid photosensitive polymeric compositions, the non-printing area of the relief elements is tacky.
In order to decrease the tackiness and increase the hardness, particularly, in the non-printing area, it is common practice to post-expose the entire surface of the plate to actinic radiation, thereby completely hardening it and rendering the surface non-tacky. Since oxygen in the atmosphere tends to delay the post-exposure photoreaction, it is most desirable to exclude oxygen. Various methods have been proposed for the exclusion of oxygen. One method consists of conducting the post-exposure in an inert atmosphere, such as nitrogen or carbon dioxide. Another method consists of covering the surface with some type of transparent film. Another method is to coat the plate with a film or membrane-forming solution or emulsion, such as an aqueous solution of gum arabic or carboxymethyl cellulose. Still another method (as shown in U.S. Pat. No. 3,723,120) involves post-exposing a photopolymer plate under water. Unfortunately, all of the above methods suffer serious drawbacks. The method of post-exposing under water, for example, requires an excessive amount of time to reduce tack.
More recently, it has become known that these drawbacks can be partially overcome by post exposing a photopolymer plate under a dilute aqueous solution of a persulfate salt. However, even where a persulfate salt solution is employed, the tack reduction is slow, erratic and incomplete.